1. Field of the Invention
The present invention relates to interchangeable lens units for use in camera systems such as video cameras, still cameras and the like.
2. Description of the Related Art
FIG. 1 shows a generally known type of interchangeable lens unit for use in a video camera system.
The illustrated lens unit is provided with a photographic optical system constructed of a focusing lens group (hereinafter referred to simply as focusing lens) L1, a zoom lens group (hereinafter referred to simply as zoom lens) L2, and a relay lens group (not shown).
A distance ring 101 having a cam portion around its outer periphery is fixed by a mounting screw 102 to a front tube 106 which retains the focusing lens L1. The front tube 106 is coupled to the outer periphery of a fixed tube 103 by helicoid thread means. A motor 105 is fixed to a structure member 104 mounted on the fixed tube 103. When the output gear 105a of the motor 105 is meshed with an external gear 101a of the distance ring 101, the front tube 106 is caused to rotate integrally with the distance ring 101 and is guided by the helicoid thread means to cause the focusing lens L1 to travel forward or backward along the optical axis thereof.
FIG. 2(a) is an enlarged view showing the enmeshment between the output gear 105a of the aforesaid motor 105 and the external gear 101a of the distance ring 101. It is assumed here that the output gear 105a in the state shown in FIG. 2(b) rotates in the counterclockwise direction as viewed in the figure so that the counterclockwise force is transmitted to the external gear 101a on the driven side as a clockwise force. In this state, if the output gear 105a is reversed in response to a certain command, the external gear 101a follows the output gear 105a but, as shown in FIG. 2(c), the motion of the external gear 101a is delayed by a time interval corresponding to a backlash "d" between these gears and a time period which elapses during the halt of the output gear 105a. As this response delay becomes larger, a lag increases between the transmission of a command from a camera body and the operation of an element to be controlled in a lens unit, with the result that various problems will be encountered. For example, during the interval of a backlash, although the driving system is activated in accordance with control information, the driven portion may not be driven. This phenomenon would cause malfunction.
Such a backlash or the like appearing in the above-described driving mechanism occurs in any mechanism portion such as an automatic diaphragm control device, a power zooming mechanism driven by a motor and the like in the lens unit.
The problem of backlashes is further explained with illustrative reference to an automatic focus adjustment operation (AF operation). During the interval of a backlash, even if a lens drive command is sent from the camera body, the lens unit is not driven. Accordingly, even if the lens unit is driven, no variation occurs in a focus signal in an AF circuit provided in the camera body. This state resembles a state which appears when an in-focus point is reached in a peak detection control method which commonly includes the steps of extracting the component in a video signal, corresponding to the degree of focusing, and driving the lens unit so as to maximize the component. Accordingly, it is likely that the AF circuit of the camera body erroneously regards the above state as an in-focus state to stop the operation of the lens unit. When the direction of driving is reversed, a so-called dead band occurs due to a backlash. As a result, each time the direction in which the lens unit is driven is changed, the operation of the lens unit will come to a halt.